This invention relates to inductive circuits and, in particular, to video circuitry used to drive a deflection yoke for controlling an electron beam in a cathode ray tube (CRT). More specifically, the invention relates to a flyback boost circuit which enables the deflection yoke to rapidly move the beam from the end of one line trace to the beginning of a subsequent line trace during raster scanning.
A problem inherent in using inductive loads is that rapid changes in current through such a load is met with the characteristic impedance of the load. In order to change the current flow through the load, a substantial increase in potential across the load over a transient time period is necessary in order to effect the change in current rapidly. Some systems using inductive loads must provide power to the load up to a certain maximum amount for the majority of operating time and must provide a substantially increased amount of power to the load for short periods of time. Such circuits include electric motors, and electron beam deflection devices such as video systems which use a magnetic coil deflection yoke.
In certain techniques used to obtain an increase in line resolution at a given frame rate, it is necessary to reduce the time allocated for flyback. In order to accomplish this, flyback boost circuits are used, which increase power to a magnetic deflection yoke in order to cause a rapid change in the yoke's magnetic field. This is because, at higher raster line rates, the time allocated for flyback decreases. If the actual deflection of an electron beam deflected by the yoke lags behind the time allocated for flyback, the result is a distortion on the left side of a screen (assuming a left-to-right horizontal line scan). This would cause the picture to appear stretched or overlapped.
Deflection boost circuits provide the additional energy needed, typically by switching "on" high voltage across the yoke. High voltage switching must be triggered at a certain time and for a certain length of time during flyback. In order to accomplish this, boost circuits usually require high power switching electronics having circuits which control pulse delay and pulse width. One type of prior art boost circuit is triggered by a separate synchronization (sync) pulse. In that system, hardware is usually needed to delay, widen and amplify the sync pulses. Furthermore, such circuits have to be designed to work within a narrow voltage range. If the voltage range is to change, adaptations must be made for the circuit to function as intended.
A further disadvantage of prior art flyback boost circuits occurs when both raster and stroke information are to be displayed. Typically, stroke information is displayed during retrace periods or as a selected alternative to a raster display. Such stroke information involves specific controls of the deflection yoke circuitry and the prior art flyback boost circuits would have to be electrically taken out of the yoke driving amplifiers during such time as stroke writing was used in order to eliminate a degrading effect on the stroke image.
This invention has, as a principle object, decreased flyback time in order to facilitate increased resolution. It is important that circuitry used to increase flyback time not have a degrading effect on a CRT image produced by stroke writing. It is further important that such a circuit be simple enough in design as to not require separate adjustment. It is further desired that such a circuit be adaptable to a wide range of driving voltages and not be required to be provided with a separate sync pulse signal.